The proper functioning of the nervous system relies on the establishment of precise neuronal circuits. Neurotrophins are extracellular instructive cues secreted by neuronal targets that influence wiring of the nervous system by regulating survival, growth and synaptic plasticity. Despite considerable progress in understanding functions of target-derived neurotrophins, a fundamental question still remains: How do a limited repertoire of neurotrophins and their receptors orchestrate the diverse events required to establish and maintain precise neuronal circuits? We hypothesize that one mechanism by which target-derived neurotrophins influence neuronal function is by regulating neuronal expression and signaling of other secreted growth factors. Such a mechanism of crosstalk between neurotrophins and other growth factor families could potentially broaden the repertoire of neurotrophin actions in the nervous system. We have recently identified an interaction between the neurotrophin and Wnt signaling pathways during development. Wnts are a large family of secreted cysteine-rich proteins, initially characterized as morphogens during embryonic patterning. However, recent studies identify surprisingly versatile roles for Wnts in neuronal development. The overall goal of this proposal is to test the hypothesis that interactions between Wnts expressed in neurons and neurotrophins secreted by their target tissues regulate neuronal development and establishment of precise neuronal circuits. Thus, our specific aims are;(1) To test the hypothesis that neuronal expression of Wnts is regulated by target-derived neurotrophins: We found that several Wnts and their receptors are expressed in sympathetic neurons, a tractable model system for studying neuron-target interactions. Of these, Wnt-5a is robustly expressed at a developmental time when sympathetic axons are reaching final targets. The goal of this aim is to demonstrate that neuronal expression of Wnt-5a is regulated by interactions with target tissues and target-derived neurotrophins. (2) To test the hypothesis that neuronal Wnts are mediators of neurotrophin function in sympathetic neurons: We found that Wnt-5a is sufficient to recapitulate the growth-promoting effects of the neurotrophin, Nerve Growth Factor (NGF), in cultured sympathetic neurons. The goal of this aim is to demonstrate that Wnt-5a functions downstream of NGF, to mediate axonal growth required for innervation of target tissues. (3) To identify mechanisms of Wnt signaling in sympathetic neurons: We will characterize the signal transduction machinery within sympathetic neurons that couples an extracellular Wnt-5a signal to changes in the cytoskeletal machinery necessary for axonal growth and branching. Our study will provide new insight into molecular mechanisms underlying neuronal development and allow for new strategies for promoting growth and regeneration of neurons following injury or disease.